The Science of Violin Making - Engineering & Science

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From Neville H. Fletcher and Thomas D. Rossing, The Physics of Musical Instruments, figure 3.13, p. 79. Copyright © 1991, Springer-Verlag. Above: The resonant modes of a square plate with free edges. The X mode and the ring mode are the second and third ones, respectively, in the top row. The numbers below each mode are frequencies corresponding to the modes, relative to that of the first mode. Lines represent nodes, or places that remain stationary as the plate vibrates at a given frequency. IMAGE NOT LICENSED FOR WEB USE Below: Quarter-cut (left) and slab-cut (right) sections of a log, with the look of a violin back made from each. These are the only cuts of wood used in violins; the softwood front plate is traditionally quarter-cut while the hardwood back plate may be quarter-cut or slab-cut. In either case, the longitudinal axis is in the plane of the plate. From David Boyden et al., The Violin Family (New York: W. W. Norton & Co., 1989) p. 6. 2 0 0 4 is normally maple. The arched outer surfaces are carved rather than formed by bending. The arching gives the thin front plate increased resistance to the lateral force exerted by vibrating strings, and subtly alters the modes in which both plates vibrate. The violin maker carefully “tunes” the front and back plates, tapping the plates with the knuckles, listening for the characteristic “tap tones.” The pitches of different harmonics are adjusted by scraping away material as necessary. Tuning the front and back plates is easily the most demanding part of making a violin, and takes years to master. VIOLINS AND SOUND A vibrating string alone produces almost no sound, as it is too thin to sufficiently disturb the air. Therefore, it is not the string itself, but the body of the violin, that actually generates its sound. When a string vibrates, the bridge rocks back and forth at the same frequency. The soundpost immobilizes the front plate directly beneath the right foot of the bridge, so that the right foot remains stationary, and the front plate is driven rapidly up and down by the left foot’s “pumping” motion. The bass bar, mounted lengthwise under the left foot of the bridge, reinforces the front plate and couples the upper and lower bouts so that they move together. The body of the violin has a number of resonant frequencies, or natural vibrational frequencies, at which a weak stimulus can cause large vibrations. Forced vibration of the top plate produces some amplification at any frequency, but the amount of amplification at a given frequency depends on how well it corresponds to one or more of these resonant frequencies. The bridge transmits a whole set of harmonics from a vibrating string to the front plate, and each harmonic is amplified according to the resonance generated at that frequency. The violin’s relative response levels to different frequencies create the instrument’s unique timbre by preferentially amplifying some harmonics and damping others. The resonant frequencies of the violin body as a whole depend most strongly on the resonant modes of the front and back plates. The plates themselves can be modeled most simply as two-dimensional panels, free to move at all points out to the edges. Just as a string vibrates in harmonics corresponding to standing waves on the string, a two-dimensional panel vibrates in specific resonant modes; the graphic above left shows the calculated resonant modes of a simulated square panel. In violins, there is a further complication: wood has very different mechanical properties along different axes. Its mechanical properties are determined entirely by its cell structure. Wood is made up of long, thin cells with walls composed of the polymers cellulose, hemicellulose, and lignin. Cellulose, a carbohydrate that forms long straight E N G I N E E R I N G & S C I E N C E N O . 4 31
The vibrational modes of the front plate (top) and back plate (bottom) of an unassembled violin made by a modern master craftsman are revealed by laser interferometry. Nodal areas appear white. The X modes and ring modes of both plates are highlighted in red. The numbers are the frequencies at which the reso- nances occur. chains, is the main structural component of wood. Cellulose chains usually form microfibrils, fibers consisting of groups of parallel chains held firmly together by hydrogen bonding. In wood, cellulose microfibrils lie parallel to one another in four layers, and spiral around the cell in its long direction, with different angles of spiraling in each layer. Lignin and hemicellulose, which form highly crosslinked structures, act as a “glue” that holds together the cellulose components and binds adjacent cells together. The longest dimension of each cell runs parallel to the growth of the tree trunk, in the longitudinal axis, and therefore wood has its greatest tensile strength in the longitudinal direction. As a result, wood plates must be elongated along the grain of the wood, in the direction of greatest tensile strength, in order to achieve the same types of resonant modes that are observed in an ideal square plate. Many modern-day violin makers use visualizations of resonant modes to aid in tuning a violin’s front and back plates. During construction, modal patterns in a plate can be seen by covering the surface of the plate with a fine sand and inducing mechanical vibrations at various frequencies. As the plate resonates, the sand moves about, except at the nodes, which remain stationary. The sand collects at the nodes or is bounced away, creating much the same patterns as shown above. Two particularly strong modes are the second and fifth harmonics of the plate, often referred to as the “X mode” and the “ring mode” for the shapes of their nodal patterns. These harmonics are the main components of a plate’s tap tone. Recently, a number of violin makers have recommended tuning each plate such that the ring mode sounds exactly an octave above the X mode, in 32 E N G I N E E R I N G & S C I E N C E N O . 4 2 0 0 4 order to mimic the efforts of early violin makers, who would have tuned the most prominent modes to exact musical intervals. While the theory is difficult to test, it seems highly plausible because tuning tap tones to musical intervals requires no specialized equipment and therefore could have been done by even the earliest violin makers. In addition, the idea parallels Renaissance ideals of mathematical perfection, which may well have guided the Italian violin makers of centuries past. LUMBER REDUX: ANOTHER LOOK AT WOOD Do violins actually improve with age? The acoustical properties of the wood used in their construction certainly change with the passage of years. Moisture in wood absorbs vibrational energy, converting it to heat energy by evaporation. Although the wood used in violins is already dry, minute changes in water content can have dramatic effects on violin acoustics: a 1 percent decrease in moisture content reduces damping by up to 3.5 percent. The long-term improvement of acoustical response depends mainly on the degradation of hemicellulose, the component of wood that adsorbs water most readily and degrades most dramatically over time. As hemicellulose degrades, the wood’s maximum water content decreases. Even over very short periods, the sound of a frequently played violin may noticeably improve as small amounts of water evaporate from the wood. ANALYZING CREMONA Age, however, does nothing to explain the From C. M. Hutchins, et al., “Clarification of Free Plate Tap Tones by Hologram Interferometry,” Catgut Acoustical Society Newsletter, No. 16 (1971), p.15. Reprinted with permission of the Violin Society of America.
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Page 3: Below: The parts of a violin. From
Page 7 and 8: From H. Dünnwald, “Ein Verfahren

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